Format a UK postcode to include spaces that may have been omitted.

postcode.js

function formatPostcode(postcode){
  return(
    //if character fourth from the right is not space
    postcode.slice(-4,-3) !== " "
    //insert space three characters from the right
    ?postcode.slice(0,-3) + " " + postcode.slice(-3)
    //otherwise use origional
    :postcode
  )
  // upper case
  .toUpperCase();
}